Brake fluid pressure control apparatus for vehicle

ABSTRACT

A brake fluid pressure control apparatus for a vehicle includes a parameter calculation unit configured to calculate a rollover detection parameter; and a steering maneuver determination unit configured to determine whether an abrupt steering maneuver is made. The parameter calculation unit is configured to calculate a first composition roll angle as the rollover detection parameter, by combining at a predetermined weight assignment ratio a first roll angle equivalent to an actual roll angle with a second roll angle obtained using a parameter which changes with a phase earlier than the first roll angle, and to calculate the first composition roll angle by changing the weight assignment ratio such that a weight of the second roll angle is higher when the steering maneuver determination unit determines that an abrupt steering maneuver is made than when the steering maneuver determination unit determines that the abrupt steering maneuver is not made.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the foreign priority benefit under Title 35,United States Code, §119(a)-(d) of Japanese Patent Application Nos.2011-036439, 2011-036110, and 2011-036441 filed on Feb. 22, 2011 in theJapan Patent Office, the disclosures of which are herein incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a brake fluid pressure controlapparatus for a vehicle, and more particularly to a brake fluid pressurecontrol apparatus for a vehicle, which controls brake to preventrollover of the vehicle.

As disclosed in JP2001-509448A (hereinafter referred to as PatentLiterature 1), there is known a technique for preventing rollover of avehicle by applying a braking force to at least one wheel to stabilizethe vehicle when a driver steers the vehicle into a sharp turn and thevehicle is about to roll over.

As disclosed in JP10-081215A (hereinafter referred to as PatentLiterature 2), there is blown a technique for preventing rollover of avehicle by applying braking forces only to turning outside wheels (i.e.,wheels which are on the outside during cornering) to stabilize thevehicle when a driver steers the vehicle into a sharp turn and thevehicle is about to roll over, so that a lateral friction between theroad surface and the tires can be reduced.

A vehicle is liable to roll over due to counteraction of roll,particularly when the driver steers back the steering wheel of a vehiclewhich is turning in one direction to the opposite direction so as tocause the vehicle to turn in the opposite direction. For the purpose ofstabilizing the vehicle after such a steering-back maneuver,JP2007-513002A (hereinafter referred to as Patent Literature 3)discloses a technique for applying preparatory brake to turning insidewheels (i.e., wheels which are on the inside during cornering) after thedriver executes the steering-back maneuver.

<First Drawback>

According to the technique disclosed in Patent Literature 1, anaccelerometer for measuring a lateral acceleration, a measuringinstrument for measuring a roll angle of the vehicle, and otherinstruments are used as sensors for detecting whether or not the vehicleis likely to roll over.

However, measuring physical quantities such as a lateral accelerationand a roll angle, which are equivalent to the actual roll angle of thevehicle, and then determining whether or not the vehicle is likely toroll over based on the measurement result may disadvantageously resultin a drawback that the rollover prevention control will not sufficientlyact on the vehicle if a rollover tendency of the vehicle increases moreabruptly than normal. For example, if the driver of the vehicle executesan abrupt steering maneuver or a steering-back maneuver while thevehicle is turning, the vehicle is liable to roll over due torolling-back of the vehicle generated after the steering maneuver. Itwould be desirable to provide a brake fluid pressure control apparatusfor a vehicle, which can determine the rollover tendency of the vehicleat a timing as early as possible to promptly initiate the rolloverprevention control.

<Second Drawback>

According to the technique disclosed in Patent Literature 1, therollover tendency of the vehicle is detected by comparing measurementsof various sensors indicating the roll angle or a measurement of alateral acceleration sensor which changes with the same phase as theactual roll angle with predetermined threshold values.

However, the liability to cause the vehicle to roll over highly dependson whether the roll angle is gradually increasing or abruptlyincreasing, even if the roll angle takes the same value. Therefore, ifthe rollover tendency is detected only by determining whether a valueequivalent to the roll angle reaches the predetermined threshold valueand the rollover prevention control is initiated based on thisdetermination, it is impossible to effectively perform the rolloverprevention control in the case where the roll angle abruptly increasesand the vehicle is more liable to roll over.

<Third Drawback>

According to the techniques disclosed in Patent Literatures 2 and 3, itis impossible to promptly increase braking forces at the wheels whichare on the outside after the steering-back maneuver (i.e., wheels whichare on the inside before the steering-back maneuver), when the drivermakes a steering-back maneuver while the vehicle is turning. Thetechnique disclosed in Patent Literature 3 is better than that disclosedin Patent Literature 2 because preparatory brake is applied after thesteering-back maneuver. However, it is after the steering-back maneuverthat the preparatory brake is applied to the turning inside wheels, andtherefore the braking forces at the wheels which are on the outsideafter the steering-back maneuver do not always increase sufficiently.Further, in this technique, the preparatory brake is applied for a shortperiod of time, which contributes little to reducing the vehicle speedthat is one of important factors to prevent rollover of the vehicle.

In view of the above, it would be desirable to provide a brake fluidpressure control apparatus, which can overcome one or more of the abovedrawbacks and improve the stability while driving the vehicle.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, the above firstdrawback is overcome by providing a brake fluid pressure controlapparatus for executing rollover prevention control, in which brake isapplied to at least one wheel of a vehicle at a timing when a rollovertendency of the vehicle is detected through a rollover detectionparameter while the vehicle is turning. The brake fluid pressure controlapparatus comprises: a parameter calculation unit configured tocalculate the rollover detection parameter; and a steering maneuverdetermination unit configured to determine whether or not an abruptsteering maneuver is made, wherein the parameter calculation unit isconfigured to calculate a first composition roll angle as the rolloverdetection parameter, by combining at a predetermined weight assignmentratio a first roll angle equivalent to an actual roll angle with asecond roll angle obtained using a parameter which changes with a phaseearlier than the first roll angle, and wherein the parameter calculationunit calculates the first composition roll angle by changing the weightassignment ratio such that a weight of the second roll angle is higherwhen the steering maneuver determination unit determines that an abruptsteering maneuver is made than when the steering maneuver determinationunit determines that the abrupt steering maneuver is not made.

With this configuration of the brake fluid pressure control apparatus,when the driver makes an abrupt steering maneuver, instead ofcalculating the rollover detection parameter only from the first rollangle equivalent to the actual roll angle, the parameter calculationunit calculates the first composition roll angle (rollover detectionparameter) by combining the first roll angle and the second roll anglesuch that the distribution ratio (weight) of the second roll angle whichchanges with a phase earlier than the first roll angle increases withrespect to the first roll angle. By this way, if an abrupt steeringmaneuver is made, the rollover detection parameter changes at a timingslightly earlier than the actual roll angle, with the result that therollover tendency of the vehicle can be detected at a timing earlierthan the conventional method, because the rollover tendency isdetermined based on this rollover detection parameter. Therefore, it ispossible to predict the rollover tendency of the vehicle at a timingearlier than the conventional method and thus to further improve thepostural stability of the vehicle, in the case where the rollovertendency increases rapidly as the result of an abrupt steering maneuver.It should be noted that the first roll angle equivalent to the actualroll angle is a physical quantity substantially accurately reflectingthe actual roll angle; for example, a roll angle detected by a rollangle sensor, and a roll angle calculated from a constant depending on alateral acceleration and roll characteristics of the vehicle areequivalent to the first roll angle.

The above brake fluid pressure control apparatus may further comprise asteering-back maneuver determination unit configured to determinewhether or not an abrupt steering-back maneuver is made, and wherein theparameter calculation unit is configured to calculate a secondcomposition roll angle as the rollover detection parameter, by combiningat a predetermined weight assignment ratio a third roll angle obtainedfrom a parameter which changes with a phase earlier than the first rollangle and the second roll angle with the first composition roll angle,and wherein the parameter calculation unit calculates the secondcomposition roll angle by changing the weight assignment ratio such thata weight of the third roll angle is higher when the steering-backmaneuver determination unit determines that an abrupt steering-backmaneuver is made than when the steering-back maneuver determination unitdetermines that the abrupt steering-back maneuver is not made.

With this configuration of the brake fluid pressure control apparatus,the second composition roll angle (rollover detection parameter) iscalculated by combining the third roll angle obtained from a parameterwhich changes with a phase earlier than the first roll angle and thesecond roll angle with the first roll angle. In this calculation, thedistribution ratio (weight) of the third roll angle is higher when thesteering-back maneuver determination unit determines that an abruptsteering-back maneuver is made than when the steering-back maneuverdetermination unit determines that the abrupt steering-back maneuver isnot made, with the result that the rollover tendency of the vehicle canbe determined at a timing much earlier than the conventional method.Therefore, it is possible to predict the rollover tendency of thevehicle at a timing earlier than the conventional method and thus tofurther improve the postural stability of the vehicle, in the case wherethe rollover tendency increases rapidly as the result of a steering-backmaneuver.

In the above brake fluid pressure control apparatus, the steeringmaneuver determination unit may determine that an abrupt steeringmaneuver is made, if an absolute value of a steering wheel turning speedis equal to or greater than a predetermined value, and an absolute valueof a lateral acceleration is equal to or greater than a predeterminedvalue. Instead of using the absolute value of the steering wheel turningspeed, it is possible to use a filtered absolute value of a lateralacceleration resulting from a filtering process by which a decrease ofthe absolute value of the lateral acceleration is retarded.

In the above brake fluid pressure control apparatus, the parametercalculation unit may comprise a first counter configured to increase afirst count value if the steering maneuver determination unit determinesthat an abrupt steering maneuver is made, and to decrease the firstcount value if the steering maneuver determination unit determines thatan abrupt steering maneuver is not made, and a first weight coefficientsetting unit configured to set a first weight coefficient, which isequivalent to the weight of the second roll angle, in accordance withthe first count value and in a range equal to or smaller than apredetermined upper limit value, and wherein the first counter increasesthe first count value even after the first weight coefficient reachesthe predetermined upper limit value.

With this configuration of the brake fluid pressure control apparatus,even after the first weight coefficient reaches the predetermined upperlimit value, the first count value is increased if the steering wheelturning speed is greater than a predetermined value. After that, even ifthe first counter decreases the first count value, the first weightcoefficient maintains the upper limit value until the first count valuedecreases to a value corresponding to the predetermined upper limitvalue of the first weight coefficient. It is therefore possible toimprove a rollover prevention effect after the end of an abrupt steeringmaneuver.

In the above brake fluid pressure control apparatus, signs of values maybe defined by assigning first signs respectively to a value of asteering angle when the steering wheel is turned left, values of alateral acceleration acting on the vehicle and a roll angle exhibitedwhile the vehicle is stably turning left, and a value of a roll rateexhibited when the roll angle takes a greater value due to a left turnof the vehicle, and by assigning second signs respectively to a value ofa steering angle when the steering wheel is turned right, values of alateral acceleration acting on the vehicle and a roll angle exhibitedwhile the vehicle is stably turning right, and a value of a roll rateexhibited when the roll angle takes a greater value due to a right turnof the vehicle, and the steering-back maneuver determination unitdetermines that an abrupt steering-back maneuver is made if all of thefollowing conditions are satisfied:

(1) one of values of the steering angle and the lateral acceleration hasthe first sign, while the other one of the values has the second sign;

(2) one of values of the first roll angle and a first roll ratecalculated from the first roll angle has the first sign, while the otherone of the values has the second sign, or/and one of values of thesecond roll angle and a second roll rate calculated from the second rollangle has the first sign, while the other one of the values has thesecond sign; and

(3) an absolute value of the first roll rate that satisfies the abovecondition (2) is equal to or greater than a predetermined value or/andan absolute value of the second roll rate that satisfies the abovecondition (2) is equal to or greater than a predetermined value.

Further, the parameter calculation unit may comprise a second counterconfigured to increase a second count value if the steering-backmaneuver determination unit determines that an abrupt steering-backmaneuver is made, and to decrease the second count value if thesteering-back maneuver determination unit determines that an abruptsteering-back maneuver is not made, and a second weight coefficientsetting unit configured to set a second weight coefficient, which isequivalent to the weight of the third roll angle, in accordance with thesecond count value and in a range equal to or smaller than apredetermined upper limit value, and wherein the second counterincreases the second count value even after the second weightcoefficient reaches the predetermined upper limit value.

With this configuration of the brake fluid pressure control apparatus,even after the second weight coefficient reaches the predetermined upperlimit value, the second count value is increased if an abruptsteering-back maneuver is made. After that, even if the second counterdecreases the second count value, the second weight coefficientmaintains the upper limit value until the second count value decreasesto a value corresponding to the predetermined upper limit value of thesecond weight coefficient. It is therefore possible to improve arollover prevention effect, particularly after the end of asteering-back maneuver.

In the above brake fluid pressure control apparatus, for example, thefirst roll angle is calculated from a lateral acceleration, and thesecond roll angle is calculated from a yaw rate. Further, the third rollangle may be calculated from a steering angle.

According to a second aspect of the present invention, the above seconddrawback is overcome by providing a brake fluid pressure controlapparatus for executing rollover prevention control, in which brake isapplied to at least one wheel of a vehicle at a timing when a rollovertendency of the vehicle is detected from a rollover detection parametergreater than a parameter threshold value while the vehicle is turning.The brake fluid pressure control apparatus comprises: a parameteracquisition unit configured to acquire the rollover detection parameter;and a parameter calculation unit configured to set the parameterthreshold value, wherein the parameter acquisition unit acquires a rollangle of the vehicle as the rollover detection parameter, and whereinthe parameter calculation unit calculates a threshold calculation rollrate, which is a rate of change of the roll angle of the vehicle, andsets the parameter threshold value to a smaller value with an increasein the threshold calculation roll rate.

With this configuration of the brake fluid pressure control apparatus,the parameter calculation unit sets the parameter threshold value to asmaller value with an increase in the threshold calculation roll rate.In the case where the threshold calculation roll rate is greater, theparameter threshold value is smaller accordingly, so that the rolloverdetection parameter is apt to be greater than the parameter thresholdvalue. As a result, the rollover prevention control can be initiatedpromptly under such a condition that the vehicle is apt to roll over dueto rapidly increased roll angle. This can improve the stability whiledriving the vehicle.

It should be noted that the roll angle obtained by the parameteracquisition unit may be a value obtained by measurement of a roll anglesensor, or a roll angle as an estimate value obtained by calculationfrom other parameters such as a lateral acceleration, a yaw rate, and asteering angle.

The above brake fluid pressure control apparatus may further comprisesan instability level calculation unit configured to calculate aninstability level which shows a greater value when a running conditionof the vehicle is unstable, and the parameter calculation unit may setthe threshold calculation roll rate to zero if the instability level issmaller than a predetermined value.

With this configuration of the brake fluid pressure control apparatus,the threshold calculation roll rate is set to zero when the instabilitylevel is smaller than the predetermined value, namely when the vehiclebehavior is not so unstable, with the result that the parameterthreshold value increases to prevent unnecessary rollover preventioncontrol.

The above brake fluid pressure control apparatus may further comprises asteering maneuver determination unit configured to determine whether ornot an abrupt steering maneuver is made, and the parameter calculationunit may be configured to calculate a composition roll rate as thethreshold calculation roll rate, by combining at a predetermined weightassignment ratio a first roll rate which is a rate of change of a firstroll angle equivalent to an actual roll angle with a second roll ratewhich is a rate of change of a second roll angle obtained using aparameter which changes with a phase earlier than the first roll angle,and to calculate the composition roll rate by changing the weightassignment ratio such that a weight of the second roll rate is higherwhen the steering maneuver determination unit determines that an abruptsteering maneuver is made than when the steering maneuver determinationunit determines that the abrupt steering maneuver is not made.

With this configuration of the brake fluid pressure control apparatus,when the driver makes an abrupt steering maneuver, instead ofcalculating the parameter threshold value only from the first roll ratewhich is a rate of change of the first roll angle equivalent to theactual roll angle, the parameter calculation unit calculates thecomposition roll rate (i.e., threshold calculation roll rate forcalculating the parameter threshold value) by increasing thedistribution ratio (weight) of the second roll rate which is a rate ofchange of the second roll angle obtained using a parameter which changeswith a phase earlier than the first roll angle, and then combining thefirst roll rate and the second roll rate using this distribution ratio.By this way, if an abrupt steering maneuver is made, the parameterthreshold value changes at a timing slightly earlier than the actualroll angle, with the result that the rollover tendency of the vehiclecan be detected at a timing earlier than the conventional method,because the rollover tendency is determined based on this parameterthreshold value. Therefore, it is possible to predict the rollovertendency of the vehicle at a timing earlier than the conventional methodand thus to further improve the postural stability of the vehicle, inthe case where the rollover tendency increases rapidly as the result ofan abrupt steering maneuver. It should be noted that the first rollangle equivalent to the actual roll angle is a physical quantitysubstantially accurately reflecting the actual roll angle: for example,a roll angle detected by a roll angle sensor, and a roll anglecalculated from a constant depending on a lateral acceleration and rollcharacteristics of the vehicle are equivalent to the first roll angle.

In the above brake fluid pressure control apparatus, the first rollangle may be calculated from a lateral acceleration, and the second rollangle may be calculated from a yaw rate.

With this configuration of the brake fluid pressure control apparatus,the first roll angle and the second roll angle are obtained usingmeasurements from sensors normally equipped in the brake fluid pressurecontrol apparatus.

In the above brake fluid pressure control apparatus, the steelingmaneuver determination unit may determine that an abrupt steeringmaneuver is made, if an absolute value of a steering wheel turning speedis equal to or greater than a predetermined value, and an absolute valueof a lateral acceleration is equal to or greater than a predeterminedvalue. Instead of using the absolute value of the steering wheel turningspeed, it is possible to use a filtered absolute value of a lateralacceleration resulting from a filtering process by which a decrease ofthe absolute value of the lateral acceleration is retarded.

In the above brake fluid pressure control apparatus, the parametercalculation unit may comprise a counter configured to increase a countvalue if the steering maneuver determination unit determines that anabrupt steering maneuver is made, and to decrease the count value if thesteering maneuver determination unit determines that an abrupt steeringmaneuver is not made, and a weight coefficient setting unit configuredto set a weight coefficient, which is equivalent to the weight of thesecond roll rate, in accordance with the count value and in a rangeequal to or smaller than a predetermined upper limit value. The countermay increase the count value even after the weight coefficient reachesthe predetermined upper limit value.

With this configuration of the brake fluid pressure control apparatus,even after the weight coefficient reaches the predetermined upper limitvalue, the count value is increased if the steering wheel turning speedis greater than a predetermined value. After that, even if the counterdecreases the count value, the weight coefficient maintains the upperlimit value until the count value decreases to a value corresponding tothe predetermined upper limit value of the weight coefficient. It istherefore possible to improve a rollover prevention effect after the endof an abrupt steering maneuver.

In the above brake fluid pressure control apparatus, the parametercalculation unit may calculate a threshold calculation roll rate, whichis a rate of change of the roll angle of the vehicle, and set theparameter threshold value to a smaller value with an increase in afiltered absolute value of the threshold calculation roll rate resultingfrom a filtering process by which a decrease of the absolute value ofthe threshold calculation roll rate is retarded.

According to a third aspect of the present invention, the above thirddrawback is overcome by providing a brake fluid pressure controlapparatus capable of applying brake individually to right and leftwheels of a vehicle arranged on the same axle, and configured to executerollover prevention control, in which brake is applied to at least onewheel of the vehicle at a timing when a rollover tendency of the vehicleis detected while the vehicle is turning. If the rollover tendency isdetected, the brake fluid pressure control apparatus operates such thata first braking force is applied to a tuning outside wheel and at thesame time a second braking force smaller than the first braking force isapplied to a turning inside wheel arranged on the same axle and that anapplication of the first braking force is initiated at the same timingas an application of the second braking force

With this configuration of the brake fluid pressure control apparatus,if the rollover tendency of the vehicle is detected, the first brakingforce is applied to the turning outside wheel, and “at the same time,”the second braking force smaller than the first braking force is appliedto the turning inside wheel (hereinafter simply referred to as an“inside wheel”) arranged on the same axle, and the application of thefirst braking force is initiated at the same timing as the applicationof the second braking force, with the result that the turning outsidewheel (hereinafter simply referred to as an “outside wheel”) receives agreater braking force than the turning inside wheel does. This canrestrict the rollover tendency irrespective of whether or not subsequentsteering-back maneuver is made. And even if a subsequent steering-backmaneuver is made, the brake pressure on the outside wheel can bepromptly increased after the steering-back maneuver, so that a rolloverof the vehicle can be effectively prevented. This is because a certainlevel of braking force has been applied to the turning inside wheel(turning outside wheel after the steering-back maneuver).

Braking force is applied not only to the outside wheel but also to theinside wheel before the steering-back maneuver, so that the vehiclespeed is effectively reduced to prevent the rollover of the vehicle.Namely, the roll angle directly indicating the rollover tendency dependson the vehicle speed and is apt to be greater with an increase in thevehicle speed. Therefore, reducing the vehicle speed can serve toprevent the rollover of the vehicle. Further, with this configuration,the vehicle speed is reduced before the steering-back maneuver is made,by making use of the braking force applied to the inside wheel. It istherefore possible to effectively prevent the rollover of the vehicleafter the driver executes the steering-back maneuver.

The above brake fluid pressure control apparatus may further comprise anoutside wheel target braking force setting unit configured to set anoutside wheel target braking force as a target for braking the turningoutside wheel with the first braking force, and an inside wheel targetbraking force setting unit configured to set an inside wheel targetbraking force as a target for braking the turning inside wheel with thesecond braking force, using a value smaller than that of the outsidewheel target braking force.

In the above brake fluid pressure control apparatus, the rolloverprevention control may be carried out if a rollover detection parameterindicating the rollover tendency is greater than a predeterminedthreshold value, and the outside wheel target braking force setting unitmay set the outside wheel target braking force based on a differencebetween the rollover detection parameter and the predetermined thresholdvalue.

The greater the difference between the rollover detection parameterindicating the rollover tendency of the vehicle and the predeterminedthreshold value for determining whether or not the rollover preventioncontrol is carried out, the higher the possibility of the rollover ofthe vehicle is. Therefore, setting the outside wheel target brakingforce based on this difference makes it possible to perform the rolloverprevention control in accordance with the possibility of rollover.

In the above brake fluid pressure control apparatus, if a predeterminedvalue is smaller than a multiplication value obtained by multiplying theoutside wheel target braking force by a coefficient smaller than 1, theinside wheel target braking force setting unit may set the predeterminedvalue as the inside wheel target braking force, and if the predeterminedvalue is not smaller than the multiplication value, the inside wheeltarget braking force setting unit may set the multiplication value asthe inside wheel target braking force.

With this configuration of the brake fluid pressure control apparatus,since the inside wheel target braking force is generally set to thepredetermined value, it is possible to prevent a delay of increase inthe braking force applied to the outside wheel after the steering-backmaneuver, and hence a deceleration performance of the vehicle can beexecuted. If the braking force applied to the outside wheel is small,the braking force on the inside wheel is set to be smaller than that ofthe outside wheel, so that a rollover prevention can be effectivelyperformed in this case.

In the above brake fluid pressure control apparatus, the inside wheeltarget braking force setting unit may set a first predetermined value asthe predetermined value for a predetermined period of time frominitiating the rollover prevention control, and after an elapse of thepredetermined period of time, sets a second predetermined value smallerthan the first predetermined value as the predetermined value.

With this configuration of the brake fluid pressure control apparatus,the inside wheel target braking force is set to a greater value at aninitial stage of the rollover prevention control. This can improve arise time of the braking force and stabilize the vehicle.

In the above brake fluid pressure control apparatus, the rolloverprevention control may be performed on both front and rear wheels, andthe outside wheel target braking force setting unit may set an outsidewheel target braking force for the rear wheels such that the outsidewheel target braking force is set within a third predetermined value.

With this configuration of the brake fluid pressure control apparatus,since braking forces are applied to front and rear wheels, the vehiclespeed can be reduced promptly to improve the rollover preventioncontrol. Further, since the upper limit value is provided for thebraking force on the rear-side outside wheel which is liable to slip, aninitiation of unnecessary braking force can be avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and aspects of the present invention will become moreapparent by describing in detail illustrative, non-limiting embodimentsthereof with reference to the accompanying drawings, in which:

FIG. 1 is a block diagram of a vehicle in which a brake fluid pressurecontrol apparatus according to one embodiment of the present inventionis installed;

FIG. 2 is a diagram showing a fluid pressure unit of the brake fluidpressure control apparatus;

FIG. 3 is a block diagram illustrating the configuration of acontroller;

FIG. 4A is a block diagram illustrating the configuration of a parametercalculation unit;

FIG. 4B is a block diagram illustrating the configuration of a ROMcontrol braking force setting unit;

FIG. 5 is a graph showing the relationship between threshold calculationroll rate and roll angle threshold value;

FIG. 6 is a flow chart explaining an overall process of a rolloverprevention control;

FIG. 7 is a flow chart explaining a process for calculating acomposition roll angle;

FIG. 8 is a flow chart explaining a process for calculating a firstweight coefficient;

FIG. 9 is a flow chart explaining a process for calculating a secondweight coefficient;

FIG. 10 is a flow chart explaining a process for setting a roll anglethreshold value;

FIG. 11 is a flow chart explaining a process for calculating an outsidewheel target braking force;

FIG. 12 is a flow chart explaining a process for calculating an insidewheel target braking force;

FIG. 13 explains a process for calculating a first composition rollangle, and shows time charts of various parameters;

FIG. 14 explains a process for calculating a second composition rollangle, and shows time charts of various parameters;

FIG. 15 explains a process for calculating a composition roll rate, andshows time charts of various parameters;

FIG. 16 shows graphs explaining the calculation of the roll anglethreshold value from the composition roll rate;

FIG. 17 shows graphs explaining the calculation of a difference ΔRa;

FIG. 18A shows graphs explaining the calculation of a PI output valuefrom the difference ΔRa;

FIG. 18B shows time charts of outside wheel target braking forces forfront wheels and rear wheels, respectively;

FIG. 19 shows time charts of the inside wheel target braking force;

FIG. 20 shows time charts of the target braking forces for the frontright wheel (a), the front left wheel (b), the rear right wheel (c), andthe rear left wheel (d), respectively; and

FIG. 21 shows time charts of a weight coefficient, a rollover detectionparameter, a caliper pressure, and a wheel lifting amount, as exhibitedwhen the rollover prevention control is carried out by the brake fluidpressure control apparatus according to one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the attached drawings, a preferred embodiment of thepresent invention will be described.

As seen in FIG. 1, a brake fluid pressure control apparatus A for avehicle CR controls a braking force (brake fluid pressure) applied toeach wheel W of the vehicle CR where appropriate. The brake fluidpressure control apparatus A comprises a fluid pressure unit 10 in whichbrake fluid passages (fluid pressure passages) and various parts areprovided, and a controller 100 for appropriately controlling the variousparts within the fluid pressure unit 10.

Connected to the controller 100 of the brake fluid pressure controlapparatus A are wheel speed sensors 91 each for detecting wheel speed ofa wheel W, a steering angle sensor 92 for detecting steering angle of asteering wheel ST, a lateral acceleration sensor 93 for detectingacceleration acting in a lateral direction of the vehicle CR (i.e.,lateral acceleration), and a yaw rate sensor 94 for detecting turningangular velocity (i.e., actual yaw rate) of the vehicle CR. Thesesensors 91-94 output detection signals to the controller 100.

The controller 100 includes a CPU, a RAM, a ROM, an input/outputcircuit, etc. The controller 100 performs various arithmetic operationsbased on input signals from the wheel speed sensors 91, the steeringangle sensor 92, the lateral acceleration sensor 93, and the yaw ratesensor 94, and also based on programs or data stored in the ROM, so asto execute the control.

A wheel cylinder H is a hydraulic device which converts brake fluidpressure generated in a master cylinder MC and the brake fluid pressurecontrol apparatus A into actuating force for a wheel brake FR, FL, RR,RL of each wheel W. In this exemplary embodiment, four wheel cylinders Hare connected to the fluid pressure unit 10 of the brake fluid pressurecontrol apparatus A through piping.

As seen in FIG. 2, the fluid pressure unit 10 of the brake fluidpressure control apparatus A is arranged between the master cylinder MCand the wheel brakes FR, FL, RR, RL. In the master cylinder MC, brakefluid pressure which varies with the driver's brake pedal depressionforce is generated. The fluid pressure unit 10 includes a pump body 10 awhich is a base body having brake fluid passages for brake fluid, aplurality of inlet valves 1 and outlet valves 2 arranged on the brakefluid passages, and other components.

Two output ports M1, M2 of the master cylinder MC are connected to twoinput ports 12A of the pump body 10 a, and output ports 12B of the pumpbody 10 a are connected to the wheel brakes FL, RR, RL, FR. Normally,the brake fluid passages from the input ports 12A to the output ports12B within the pump body 10 a provide a fluid communication so that whenthe driver depresses a brake pedal BP, the brake pedal depression forceis transmitted to the wheel brakes FL, RR, RL, FR.

Herein, the brake fluid passage extending from the output port M1 to thewheel brake FL at the front left wheel and the wheel brake RR at therear right wheel is referred to as a “first brake system” whereas thebrake fluid passage extending from the output port M2 to the wheel brakeFR at the front right wheel and the wheel brake RL at the rear leftwheel is referred to as a “second brake system.”

The fluid pressure unit 10 includes two control valve units V in thefirst brake system corresponding to the wheel brakes FL, RR, andsimilarly two control valve units V in the second brake systemcorresponding to the wheel brakes RL, FR. In this fluid pressure unit10, each of the first and second brake systems includes a reservoir 3, apump 4, an orifice 5 a, a pressure regulating valve (regulator) R, and asuction valve 7. Further, the fluid pressure unit 10 includes a motor 9for driving both the pump 4 in the first brake system and the pump 4 inthe second brake system. The motor 9 is of a speed control type. In thisexemplary embodiment, a pressure sensor 8 is provided only in the secondbrake system.

In the following description, the brake fluid passage extending from theoutput port M1, M2 of the master cylinder MC to each pressure regulatingvalve R is referred to as an “output fluid pressure passage A1”, and thebrake fluid passage extending from the pressure regulating valve R inthe first brake system to the wheel brakes FL, RR and the brake fluidpassage extending from the pressure regulating valve R in the secondbrake system to the wheel brakes RL. FR are both referred to as a “wheelfluid pressure passage B.” The brake fluid passage extending from theoutput fluid pressure passage A1 to the pump 4 is referred to as a“suction fluid pressure passage C”, the brake fluid passage extendingfrom the pump 4 to the wheel fluid pressure passage B is referred to asa “discharge fluid pressure passage D”, and the brake fluid passageextending from the wheel fluid pressure passage B to the suction fluidpressure passage C is referred to as a “release passage E.”

The control valve unit V controls a flow of fluid from the mastercylinder MC or the pump 4 to the wheel brakes FL, RR, RL, FR(specifically, the wheel cylinders H) and vice versa, so that thepressure within the wheel cylinder H can be increased, maintained ordecreased. For this purpose, the control valve unit V includes an inletvalve 1, an outlet valve 2, and a check valve 1 a.

The inlet valve 1 is a normally open solenoid valve provided betweeneach of the wheel brakes FL, RR, RL, FR and the master cylinder MC,namely in the wheel fluid pressure passage B. The inlet valve 1 is anormally open to allow transmission of brake fluid pressure from themaster cylinder MC to each of the wheel brakes FL, FR, RL, RR. However,if the wheel W is almost locked, the inlet valve 1 is closed undercontrol of the controller 100 to shut off the transmission of brakefluid pressure from the brake pedal BP to each of the wheel brakes FL,FR, RL, RR.

The outlet valve 2 is a normally closed solenoid valve provided betweeneach of the wheel brakes FL, RR, RL, FR and the reservoirs 3, namelybetween the wheel fluid pressure passage B and the release passage E.The outlet valve 2 is normally closed. However, if the wheel W is almostlocked, the outlet valve 2 is opened under control of the controller 100to release the brake fluid pressure acting on each of the wheel brakesFL, FR, RL, RR to the reservoirs 3.

The check valve 1 a is connected in parallel to each of the inlet valves1. The check valve 1 a is a one-way valve which only allows a flow ofbrake fluid from each of the wheel brakes FL, FR, RL, RR to the mastercylinder MC. When the brake pedal depression force is released at thebrake pedal BP, even if the inlet valve 1 is closed, the check valve 1 aallows a flow of brake fluid from each wheel brake FL, FR, RL, RR to themaster cylinder MC.

The reservoir 3 is provided in the release passage E, and when each ofthe outlet valves 2 is released, brake fluid released from the outletvalve 2 flows into the reservoir 3 to absorb brake fluid pressure.Provided between the reservoir 3 and the pump 4 is a check valve 3 a,which only allows a flow of brake fluid from the reservoir 3 to the pump4.

The pump 4 is provided between the suction fluid pressure passage C incommunication with the output fluid pressure passage A1 and thedischarge fluid pressure passage D in communication with the wheel fluidpressure passage B. The pump 4 sucks brake fluid stored in the reservoir3 and feeds the brake fluid to the discharge fluid pressure passage D.Therefore, the pump 4 can return the brake fluid stored in the reservoir3 to the master cylinder MC and can generate brake fluid pressure sothat a braking force is generated at each of the wheel brakes FL, RR,RL, FR without requiring the brake pedal operation of the driver.

The pump 4 discharges brake fluid at a predetermined discharge ratewhich depends on the rotation speed of the motor 9. To be more specific,the pump 4 discharges brake fluid at a greater discharge rate as therotation speed of the motor 9 increases.

The orifice 5 a operates to attenuate pressure pulsation of the brakefluid discharged from the pump 4 as well as to attenuate pulsationgenerated upon actuation of the pressure regulating valve R to bedescribed later.

The pressure regulating valve R is normally open to allow a flow ofbrake fluid from the output fluid pressure passage A1 to the wheel fluidpressure passage B. However, when the fluid pressure within the wheelcylinder H is to be increased using the brake fluid pressure generatedby the pump 4, the pressure regulating valve R shuts off the flow ofbrake fluid and regulates fluid pressure in the discharge fluid pressurepassage D, the wheel fluid pressure passage B and the wheel cylinder Hto be equal to or lower than a setting value. For this purpose, thepressure regulating valve R includes a changeover valve 6 and a checkvalve 6 a.

The changeover valve 6 is a normally open linear solenoid valve providedbetween the output fluid pressure passage A1 in communication with themaster cylinder MC and the wheel fluid pressure passage B incommunication with each of the wheel brakes FL, FR, RL, RR. Although notshown in detail in the drawings, the valve element of the changeovervalve 6 is urged toward the wheel fluid pressure passage B and the wheelcylinder H by an electromagnetic force which varies with an electriccurrent applied under control of the controller 100. When the fluidpressure in the wheel fluid pressure passage B increases to or beyond apredetermined value (the value being determined based on the appliedelectric current) which is higher than the fluid pressure in the outputfluid pressure passage A1, brake fluid is released from the wheel fluidpressure passage B to the output fluid pressure passage A1 through thechangeover valve 6, so that the fluid pressure within the wheel fluidpressure passage B is adjusted to a predetermined pressure.

The check valve 6 a is connected in parallel to each of the changeovervalves 6. The check valve 6 a is a one-way valve for allowing a flow ofbrake fluid from the output fluid pressure passage A1 to the wheel fluidpressure passage B.

The suction valve 7 is a normally closed solenoid valve provided in thesuction fluid pressure passage C so as to switch between a state wherethe suction fluid pressure passage C is opened and a state where thesuction fluid pressure passage C is closed. The suction valve 7 isreleased (open) under control of the controller 100 when the changeovervalve 6 is closed, that is, when brake fluid pressure is to be appliedto each of the wheel brakes FL, FR, RL, RR while the driver does notoperate the brake pedal BP.

The pressure sensor 8 detects brake fluid pressure in the output fluidpressure passage A1 in the second brake system. Detection results of thepressure sensor 8 are input to the controller 100.

Next, the controller 100 will be described in detail. As seen in FIG. 3,the controller 100 controls opening/closing operations of the controlvalve unit V, the changeover valve 6 (pressure regulating valve R) andthe suction valve 7 in the fluid pressure unit 10 as well as theoperation of the motor 9, based on the input signals from the sensors91-94, to thereby control the operation of each of the wheel brakes FL,RR, RL, FR. The controller 100 includes as functional units a behaviorcontrol unit 110, a rollover prevention control unit 120, a targetbraking force setting unit 130, a valve drive unit 140, a motor driveunit 150, and a storage unit 180. The storage unit 180 storespredetermined constants, values detected by the sensors, and valuescalculated by the functional units, according to necessity.

The behavior control unit 110 is a known controller for stabilizing thebehavior of the vehicle CR. The behavior control omit 110 executes acontrol for applying a braking force to one or more of the four wheelsW. Braking forces calculated by the behavior control unit 110 andapplied to the respective wheels W are output to the target brakingforce setting unit 130.

The rollover prevention control unit 120 is configured to execute arollover prevention control, when a rollover tendency of the vehicle CRis detected through a rollover detection parameter while the vehicle CRis turning (cornering); the rollover prevention control is performed byapplying a braking force to at least one wheel (in this embodiment,braking forces are applied individually to right and left wheels Warranged on the same axle). To be more specific, if the rollovertendency is detected, the rollover prevention control unit 120 operatessuch that a first braking force is applied to a turning outside wheeland at the same time a second braking force smaller than the firstbraking force is applied to a turning inside wheel arranged on the sameaxle and that an application of the first braking force is initiated atthe same timing as an application of the second braking force. For thispurpose, the rollover prevention control unit 120 includes a roll anglecalculation unit 121, a roll rate calculation unit 122, a correctedlateral acceleration calculation unit 123, a steering wheel turningspeed calculation unit 124, a steering maneuver determination unit 125,a steering-back maneuver determination unit 126, an instability levelcalculation unit 127, a parameter calculation unit 128, and a ROM(Rollover Mitigation) control braking force setting unit 129.

The roll angle calculation unit 121 calculates a lateralacceleration-based roll angle Ra1 as an example of a first roll angle, ayaw rate-based roll angle Ra2 as an example of a second roll angle, anda steering angle-based roll angle Ra3 as an example of a third rollangle. Calculation methods for these roll angles are already known inthe art, and the roll angles can be calculated by the followingexpressions:Ra1=(Hg×W×Yg)/(Gf+Gr)Ra2=(Hg×ω×Vx)/(Gf+Gr)Ra3=(Hg×W×ω _(θ) ×Vx)/(Gf+Gr)where Hg is a distance in the vertical direction between a roll axis andthe center of gravity; W is a sprung weight; Gf and Gr represent a rollstiffness; Yg represents a lateral acceleration; ω represents a yawrate; ω_(θ) represents a standard yaw rate (the standard yaw rate iscalculated based on a steering angle and a vehicle speed Vx); and Vxrepresents a vehicle speed.

The roll rate calculation unit 122 calculates a lateralacceleration-based roll rate Ra1′ from the lateral acceleration-basedroll angle Ra1. The roll rate calculation unit 122 also calculates a yawrate-based roll rate Ra2′ from the yaw rate-based roll angle Ra2. Eachof these roll rates can be obtained by calculating the rate of change intime of these roll angles.

The corrected lateral acceleration calculation unit 123 calculates acorrected lateral acceleration Ygm as a value for evaluating the lateralacceleration Yg; the corrected lateral acceleration Ygm is a filteredabsolute value of the lateral acceleration Yg resulting from a filteringprocess by which a decrease of the absolute value of the lateralacceleration Yg is retarded. To be more specific, the calculation ismade by taking the absolute value of the lateral acceleration Yg andthen changing a value of the corrected lateral acceleration Ygm to besmaller than the previous value in a range of a predetermined rate ofchange such that, if the absolute value of the lateral acceleration Ygis increasing, the corrected lateral acceleration Ygm takes the samevalue as that of the absolute value of the lateral acceleration Yg, andif the absolute value of the lateral acceleration Yg is decreasing, thecorrected lateral acceleration Ygm decreases less. See the graph showingcorrected lateral acceleration of FIG. 13.

The steering wheel turning speed calculation unit 124 calculates therate of change of the steering angle δ and then works out the steeringwheel turning speed δ′ by filtering the calculated value.

The steering maneuver determination unit 125 determines whether or notthe driver executes an abrupt steering maneuver. To be more specific,the steering maneuver determination unit 125 determines that an abruptsteering is made, if the absolute value of the steering wheel turningspeed δ′ is equal to or greater than a predetermined value δ′th and theabsolute value of the corrected lateral acceleration Ygm is equal to orgreater than a predetermined value Ygth.

The steering-back maneuver determination unit 126 determines whether ornot the driver executes an abrupt steering-back maneuver. To be morespecific, the steering-back maneuver determination unit 126 determinesthat an abrupt steering-back maneuver is made, if all of the followingconditions are satisfied.

(1) Signs indicating right and left of the steeling angle δ aredifferent from signs indicating right and left of the lateralacceleration Yg. Namely, supposing that signs of values are defined byassigning first signs (e.g., left) respectively to a value of thesteering angle when the steering wheel is turned left, values of thelateral acceleration acting on the vehicle and the roll angle exhibitedwhile the vehicle is stably turning left, and a value of the roll rateexhibited when the roll angle takes a greater value due to a left trueof the vehicle, and by assigning second signs (e.g., right) respectivelyto a value of the steering angle when the steering wheel is turnedright, values of the lateral acceleration acting on the vehicle and theroll angle exhibited while the vehicle is stably turning right, and avalue of the roll rate exhibited when the roll angle takes a greatervalue due to a right turn of the vehicle, one of values of the steeringangle δ and the lateral acceleration Yg has the first sign while theother one of the values has the second sign.

(2) One of values of the lateral acceleration-based roll angle Ra1 andthe lateral acceleration-based roll rate Ra1′ has the first sign whilethe other one of the values has the second sign.

(3) One of values of the yaw rate-based roll angle Ra2 and the yawrate-based roll rate Ra2′ has the first sign while the other one of thevalues has the second sign.

(4) The absolute value of the lateral acceleration-based roll rate Ra1′is equal to or greater than a predetermined value Ra1′ th.

(5) The absolute value of the yaw rate-based roll rate Ra2′ is equal toor greater than a predetermined value Ra2′ th.

The instability level calculation unit 127 calculates an instabilitylevel of the running condition of the vehicle CR using a conventionallyknown method, based on an actual yaw rate Ys obtained with aconventional filtering technique by filtering an actual yaw ratedetected by the yaw rate sensor 94 and a standard yaw rate Yc obtainedwith a conventional method from the steering angle δ and the vehiclespeed Vx. To be more specific, the instability level calculation unit127 obtains the instability level by filtering the difference betweenthe actual yaw rate Ys and the standard yaw rate Yc (i.e., the result ofthe subtraction between the actual yaw rate Ys and the standard yaw rateYc). The instability level shows a greater value when the runningcondition of the vehicle CR is unstable.

The parameter calculation unit 128 calculates a value (parameter)indicating the rollover tendency based on the values calculated by thevarious units described above. The parameter calculation unit 128 alsocalculates a threshold calculation roll rate which is a rate of changeof the roll angle of the vehicle CR, and sets a roll angle thresholdvalue Rath as a parameter threshold value to a smaller value with anincrease in the threshold calculation roll rate.

The parameter calculation unit 128 calculates a first composition rollangle Ra12 by combining at a predetermined weight assignment ratio afirst roll angle (the lateral acceleration-based roll angle Ra1)equivalent to the actual roll angle with a second roll angle (the yawrate-based roll angle Ra2) obtained using a parameter which changes witha phase earlier than the first roll angle, and calculates the firstcomposition roll angle Ra12 by changing the weight assignment ratio suchthat a weight of the second roll angle is higher when the steeringmaneuver determination unit 125 determines that an abrupt steeringmaneuver is made than when the steering maneuver determination unit 125determines that the abrupt steering maneuver is not made.

The parameter calculation unit 128 also calculates a second compositionroll angle Ra as the rollover detection parameter, by combining at apredetermined weight assignment ratio a third roll angle (the steeringangle-based roll angle Ra3) obtained from a parameter which changes witha phase earlier than the first roll angle and the second roll angle withthe first composition roll angle Ra12, and calculates the secondcomposition roll angle Ra by changing the weight assignment ratio suchthat a weight of the third roll angle is higher when the steering-backmaneuver determination unit 126 determines that an abrupt steering-backmaneuver is made than when the steering-back maneuver determination unit126 determines that the abrupt steering-back maneuver is not made.

According to this embodiment, a first weight coefficient K1 whichchanges according to the steering wheel turning speed is used as thepredetermined weight assignment ratio for calculating the firstcomposition roll angle Ra12, and a second weight coefficient K2 whichchanges according to an abrupt steering-back maneuver is used as thepredetermined weight assignment ratio for calculating the secondcomposition roll angle Ra. To this end, as seen in FIG. 4A, theparameter calculation unit 128 includes a first counter 128A, a secondcounter 128B, a first weight coefficient setting unit 128C, and a secondweight coefficient setting unit 128D.

The first counter 128A increases (increments) a first count value C1within a range of an upper limit value C1max if the steering maneuverdetermination unit 125 determines that an abrupt steering maneuver ismade, and decreases (decrements) the first count value C1 if thesteering maneuver determination unit 125 determines that an abruptsteering maneuver is not made. Values of the increment and the decrementmay be same or different. In this embodiment, in order to leave theinfluence of an abrupt steering maneuver for a relatively longer periodof time if the driver executes the abrupt steering maneuver, the valueof the decrement is set smaller than the value of the increment. Thefirst counter 128A increases the first count value C1 even after thefirst weight coefficient K1 reaches a predetermined upper limit value tobe described later. Accordingly, even if the decrement of the firstcount value C1 is initiated after the first weight coefficient K1reaches the predetermined upper limit value, the first weightcoefficient K1 is maintained to the upper limit value until the firstcount value C1 decreases to a value corresponding to the predeterminedupper limit value of the first weight coefficient K1. It is thereforepossible to improve a rollover prevention effect after the end of theabrupt steering maneuver.

The second counter 128B increases (increments) a second count value C2within a range of an upper limit value C2max if the steering-backmaneuver determination unit 126 determines that an abrupt steering-backmaneuver is made, and decreases (decrements) the second count value C2if the steering-back maneuver determination unit 126 determines that anabrupt steering maneuver is not made. Values of the increment and thedecrement may be same or different. In this embodiment, in order toleave the influence of an abrupt steering-back maneuver for a relativelylonger period of time if the driver executes the abrupt steering-backmaneuver, the value of the decrement is set smaller than the value ofthe increment. The second counter 128B increases the second count valueC2 even after the second weight coefficient K2 reaches a predeterminedupper limit value to be described later. Accordingly, even if thedecrement of the second count value C2 is initiated after the secondweight coefficient K2 reaches the predetermined upper limit value, thesecond weight coefficient K2 is maintained to the upper limit valueuntil the second count value C2 decreases to a value corresponding tothe predetermined upper limit value of the second weight coefficient K2.It is therefore possible to improve a rollover prevention effect,particularly after the end of the steering-back maneuver.

The first weight coefficient setting unit 128C sets the first weightcoefficient K1, which is equivalent to the weight of the yaw rate-basedroll angle Ra2, in accordance with the first count value C1 and in arange equal to or smaller than the predetermined upper limit value. Thefirst weight coefficient K1 set in this stage is also used as a weightcoefficient for the yaw rate-based roll rate Ra2′. According to thisembodiment, the first weight coefficient K1 is a value obtained bymultiplying the first count value C1 by a fixed coefficient α1, and thepredetermined upper limit value is 1.

The second weight coefficient setting unit 128D sets the second weightcoefficient K2, which is equivalent to the weight of the steeringangle-based roll angle Ra3, in accordance with the second count value C2and in a range equal to or smaller than the predetermined upper limitvalue. According to this embodiment, the second weight coefficient K2 isa value obtained by multiplying the second count value C2 by a fixedcoefficient α2, and the predetermined upper limit value is K2max whichis a value smaller than 1.

The parameter calculation unit 128 calculates the first composition rollangle Ra12 and the second composition roll angle Ra by the followingexpressions, using the first weight coefficient K1 and the second weightcoefficient K2 calculated by the various units 128A-128D as describedabove.Ra12=K1×Ra2+(1−K1)×Ra1Ra=K2×Ra3+(1−K2)×Ra12

In order to set the roll angle threshold value Rath, as best seen inFIG. 4, the parameter calculation unit 128 includes a thresholdcalculation roll rate calculation unit 128E. The roll angle used forcalculating the threshold calculation roll rate may be a roll angleequivalent to the actual roll angle or an estimated roll anglecalculated from another parameter; the roll angle equivalent to theactual roll angle includes, for example, a roll angle obtained from theroll angle sensor, and a lateral acceleration-based roll angle. Thisroll angle may be the same as the rollover detection parameter orobtained separately. Further, unless the meaning of its physicalquantity is lost, a filtering process may be applied to the roll angle.Alternatively, other calculation methods may be adopted such as bycombining the roll angle with another value. In this embodiment, thelateral acceleration-based roll angle Ra1 and the yaw rate-based rollangle Ra2 calculated by the roll angle calculation unit 121 are used forcalculating the threshold calculation roll rate.

The threshold calculation roll rate calculation unit 128E calculate acomposition roll rate Ra12′ used as the threshold calculation roll rate,by combining at a predetermined weight assignment ratio the lateralacceleration-based roll rate Ra1′ which is a rate of change of the firstroll angle (the lateral acceleration-based roll angle Ra1) equivalent tothe actual roll angle with the yaw rate-based roll rate Ra2′ which is arate of change of the second roll angle (the yaw rate-based roll angleRa2) obtained using a parameter which changes with a phase earlier thanthe first roll angle. The threshold calculation roll rate calculationunit 128E calculates the composition roll rate Ra12′ by changing theweight assignment ratio such that a weight of the second roll rate ishigher when the steering maneuver determination unit 125 determines thatan abrupt steering maneuver is made than when the steering maneuverdetermination unit 125 determines that the abrupt steering maneuver isnot made.

To be more specific, the threshold calculation roll rate calculationunit 128E calculates the composition roll rate Ra12′ by the followingexpression, using the first weight coefficient K1 calculated by thefirst weight coefficient setting unit 128C.Ra12′=K1×Ra2′+(1−K1)×Ra1′

The threshold calculation roll rate calculation unit 128E sets thethreshold calculation roll rate to zero if the instability level issmaller than a predetermined value Lv.

The parameter calculation unit 128 sets the roll angle threshold valueRath, using a value of the threshold calculation roll rate set by thethreshold calculation roll rate calculation unit 128E and referring to aconversion table between threshold calculation roll rate and roll anglethreshold value Rath. The conversion table is stored in the storage unit180. As best seen in FIG. 5, in this conversion table, the roll anglethreshold value Rath becomes smaller with an increase in the thresholdcalculation roll rate. To be more specific, the roll angle thresholdvalue Rath is a fixed value ε until the threshold calculation roll ratebecomes γ1, the roll angle threshold value Rath decreases to 0 (zero) ata constant slope when the threshold calculation roll rate is in therange from γ1 to γ2, and the roll angle threshold value Rath is 0 whenthe threshold calculation roll rate is greater than γ2.

Since the threshold calculation roll rate takes a fixed value ε untilthe threshold calculation roll rate becomes γ1, it is possible toprevent unnecessary rollover prevention control, for example, when thedriver steers the vehicle into a J-turn by his slow steeling maneuver.Further, since the roll angle threshold value Rath is 0 when thethreshold calculation roll rate is greater than γ2, it is possible toreliably execute the rollover prevention control under the conditionthat the vehicle is liable to roll over.

As seen in FIG. 4B, the ROM control braking force setting unit 129includes an outside wheel target braking force setting unit 129A and aninside wheel target braking force setting unit 129B.

The outside wheel target braking force setting unit 129A sets an outsidewheel target braking force as a target for braking the turning outsidewheel with the first braking force; the outside wheel target brakingforce setting unit 129A sets the outside wheel target braking forcebased on a difference ΔRa between the second composition roll angle Raas the rollover detection parameter and the roll angle threshold valueRath. The rollover detection parameter is an index which shows a greatervalue if the possibility of the rollover of the vehicle is higher, andthe roll angle threshold value Rath is a reference value for determiningthe possibility of the rollover. Accordingly, since the greater thedifference ΔRa between the rollover detection parameter and thepredetermined threshold value (the roll angle threshold value Rath inthis embodiment), the higher the possibility of the rollover of thevehicle is, setting the braking force applied to the turning outsidewheel to a value corresponding to the difference ΔRa makes is possibleto apply an appropriate braking force to the outside wheel to preventthe rollover of the vehicle.

To be more specific, the difference ΔRa is obtained as follows. First, adifference ΔRa (left) during a left turn of the vehicle and a differenceΔRa (right) during a right turn of the vehicle are calculated by thefollowing expressions. Herein, the roll angle threshold value Rath iseither 0 or a positive value.ΔRa(left)=MAX{(Ra−Rath),0}ΔRa(right)=MAX{((−Rath)−Ra),0}

Second, of these differences ΔRa (left) and ΔRa (right), the one withgreater value is adopted as ΔRa.

Further, according to this embodiment, in order to set a moreappropriate braking force by means of a PI control, the outside wheeltarget braking force setting unit 129A calculates a PI output value fromthe difference ΔRa. In order to adjust the magnitude of the value, theoutside wheel target braking force setting unit 129A inputs a valueobtained by multiplying the PI output value by a correction coefficientas an outside wheel target braking force Fout.

Further, the outside wheel target braking force setting unit 129A setsthe outside wheel to target braking force Fout1 of the rear wheels suchthat the upper limit value of the outside wheel target braking forceFout1 is a predetermined value Foutmax (i.e., third predeterminedvalue). As compared with the front wheels, the rear wheels are liable toslip during braking. For this reason, the upper limit value is providedfor preventing an unstable posture of the vehicle CR due to slippage ofthe rear wheels.

The outside wheel target braking force is calculated based on thedifference ΔRa. If the rollover tendency is not determined afterdetermination of the rollover tendency, the outside wheel target brakingforce setting unit 129A substitutes 0 for the difference ΔRa.

The inside wheel target braking force setting unit 129E sets the insidewheel target braking force as a target for braking the turning insidewheel with the second braking force, using a value smaller than that ofthe outside wheel target braking force. In this embodiment, the insidewheel target braking force setting unit 129B basically sets apredetermined value as the inside wheel target braking force. To be morespecific, the predetermined value is set as a first predetermined valueB1 during a predetermined period of time Tm1 from initiating therollover prevention control. After an elapse of the predetermined periodof time Tm1, the predetermined value is set as a second predeterminedvalue B2 which is smaller than the first predetermined value B1.Accordingly, a rise time of the braking force can be improved, with theresult that the braking of the inside wheel acts effectively at aninitial stage of the rollover prevention control to reduce the vehiclespeed and therefore the vehicle can be stabilized because of thereduction rollover tendency.

Further, even if the outside wheel target braking force is set to asmaller value in accordance with a smaller rollover tendency, it ispreferable that the inside wheel target braking force is smaller thanthe outside wheel target braking force. For this reason, if apredetermined value (the first predetermined value B1 or the secondpredetermined value B2) is smaller than a multiplication value obtainedby multiplying the outside wheel target braking force by a coefficient βsmaller than 1, the inside wheel target braking force setting unit 129Bsets the predetermined value as the inside wheel target braking force.If the predetermined value is not smaller than the multiplication value,the inside wheel target braking force setting unit 129B sets themultiplication value as the inside wheel target braking force.Therefore, the inside wheel target braking force is always smaller thanthe multiplication value obtained by multiplying the outside wheeltarget braking force by the coefficient β which is smaller than 1. Inother words, a rollover of the vehicle CR can be prevented because thebraking force applied to the outside wheel becomes greater than thatapplied to the inside wheel. The specific process for setting the insidewheel target braking force will be described later with reference to theflow chart.

The ROM control braking force setting unit 129 determines that thevehicle CR is turning right or left based on the value of the secondcomposition roll angle Ra, and sets the target braking force for each ofthe wheels. To be more specific, the ROM control braking force settingunit 129 sets the target braking force for each of the wheels, based onthe target braking force Fout for the front-side outside wheel and thetarget braking force Fout1 for the rear-side outside wheel, which arecalculated by the outside wheel target braking force setting unit 129A,and based on the target braking force Fin for the front-side insidewheel and the target braking force Fin1 for the rear-side inside wheel,which are calculated by the inside wheel target braking force settingunit 129B.

The rollover prevention control unit 120 always monitors the secondcomposition roll angle Ra and executes the rollover prevention controlby applying a braking force to at least one of the wheel brakes FL, RR,RL, FR at a timing when the difference ΔRa between the secondcomposition roll angle Ra and the positive roll angle threshold valueRath is greater than the predetermined value ΔRath (ΔRath is either 0 ora positive value) or when the difference between the second compositionroll angle Ra and a predetermined negative roll angle threshold value−Rath is smaller than a predetermined value −ΔRath. Although thepredetermined value ΔRath used may include any arbitrary values such as0, setting the predetermined value ΔRath to an appropriate value makesit possible to prevent the rollover prevention control from beingperformed more sensitive than necessary. In this instance, the brakingforces are applied simultaneously to the right and left wheels Warranged on the same axle throughout the time that the rolloverprevention control is performed. In other words, the application of thebraking force to the right wheel is initiated at the same timing as theapplication of the braking force to the left wheel. According to thisembodiment, since braking forces are applied to both front and rearwheels during the rollover prevention control, all of the four wheelsare always braked during the rollover prevention control. In thisembodiment, even after the rollover prevention control flag is changedfrom ON to OFF, the braking force is not immediately decreased to 0. Inorder to avoid abrupt change of the braking force at the outside wheel,the braking force is gradually decreased to 0.

The target braking forces for each of the wheels W, which are set asdescribed above and used for the rollover prevention control, are outputto the target braking force setting unit 130.

In the brake fluid pressure control apparatus according to thisembodiment, the second composition roll angle Ra based on the lateralacceleration-based roll angle Ra1, the yaw rate-based roll angle Ra2 andthe steering angle-based roll angle Ra3 is used as the rolloverdetection parameter. Namely, various units for calculating the secondcomposition roll angle Ra constitute examples of a parameter acquisitionunit configured to acquire the rollover detection parameter.

The target braking force setting unit 130 compares braking forces outputfrom the behavior control unit 110 and to be applied to each of thewheels W with braking forces output from the rollover prevention controlunit 120 and to be applied to each of the wheels W to determine greaterbraking forces, and then sets the greater braking forces as the targetbraking forces for each of the wheels W. The target braking forcesetting unit 130 outputs movements of the various valves and the motor 9in the fluid pressure unit 10 to the valve drive unit 140 and the motordrive unit, respectively, in accordance with the target braking forces.

The valve drive unit 140 actually drives the control valve units V, thepressure regulating valves R, and the suction valves 7, in accordancewith instructions from the target braking force setting unit 130.

The motor drive unit 150 drives the motor 9 to rotate in accordance withinstructions from the target braking force setting unit 130. Descriptionwill be given of the process for executing the rollover preventioncontrol by the controller 100 as described above.

As best seen in FIG. 7, the controller 100 reads detection signals fromvarious sensors including the wheel speed sensors 91, the steering anglesensor 92, the lateral acceleration sensor 93, and the yaw rate sensor94 (S1). The controller 100 then calculates the composition roll angle(S100).

The composition roll angle is calculated as shown in FIG. 7. The rollangle calculation unit 121 calculates the lateral acceleration-basedroll angle Ra1, the yaw rate-based roll angle Ra2, and the steeringangle-based roll angle Ra3, based on the detection values of the sensors91-94 and the constants stored in the storage unit 180 (S102-S104).

The roll rate calculation unit 122 then calculates the lateralacceleration-based roll rate Ra1′ by way of calculating a rate of changeof the lateral acceleration-based roll angle Ra1, and calculates the yawrate-based roll rate Ra2′ by way of calculating a rate of change of theyaw rate-based roll angle Ra2 (S105). The corrected lateral accelerationcalculation unit 123 calculates the corrected lateral acceleration Ygmfrom the lateral acceleration yg. Further, the steering wheel turningspeed calculation unit 124 calculates a rate of change of the steeringangle δ and applies a filtering process to calculate the steering wheelturning speed δ′ (S106).

The first weight coefficient setting unit 128C in the rolloverprevention control unit 120 calculates the first weight coefficient K1(S200). The first weight coefficient K1 is calculated by the processshown in FIG. 8.

To be more specific, the steering maneuver determination unit 125determines whether or not the absolute value of the steering wheelturning speed δ′ is equal to or greater than the predetermined valueδ′th and the absolute value of the corrected lateral acceleration Ygm isequal to or greater than the predetermined value Ygth (S201). Referringto FIG. 13, these conditions are satisfied in the time period from t11to t13. If these conditions are satisfied, it is determined that anabrupt steering is made with a higher steering wheel turning speed δ′and a certain higher level of lateral acceleration Yg, which is likelyto cause a rollover of the vehicle. For this reason, if the conditionsin step S201 are satisfied (S201; Yes), the first counter 128A increases(increments) the first count value C1 (S202). Further, if the firstcount value C1 is greater than the upper limit value C1max (S203; Yes),the first counter 128A sets the first count value C1 to the upper limitvalue C1max (S204). If the first count value C1 is not greater than theupper limit value C1max (S203; No), the first counter 128A sets theincreased value as the first count value C1.

On the contrary, if these conditions are not satisfied in step S201(S201; No), that is, when an abrupt steeling maneuver is not made, thefirst counter 128A decreases (decrements) the first count value C1(S208, see also time period from t13 to t15 of FIG. 13): If the firstcount value C1 is smaller than 0 (S209; Yes), the first count value C1is set to 0 (S210). If the first count value C1 is not smaller than 0(S209; No), the value is set as the first count value C1.

When the first count value C1 is determined by the above steps, thefirst count value C1 is multiplied by a coefficient α1 to obtain thefirst weight coefficient K1 (S205). If the first weight coefficient K1is greater than 1 (S206; Yes), the first weight coefficient K1 is set tothe upper limit value, that is 1 in this embodiment (S207). If the firstweight coefficient K1 is not greater than 1 (S206; No), the value is setas the first weight coefficient K1.

Next, the second weight coefficient setting unit 128D in the rolloverprevention control unit 120 calculates the second weight coefficient K2(S300). The second weight coefficient K2 is calculated by the processshown in FIG. 9.

To be more specific, the steering-back maneuver determination unit 126determines in step S301 whether or not the multiplication value δ×Yg isnegative, that is, whether or not the signs indicating fight and left ofthe steering angle δ and the lateral acceleration Yg are different fromeach other (i.e., whether or not the countersteering is detected; seealso time period from t21 to t26 of FIG. 14). If the multiplicationvalue δ×Yg is negative (S301; Yes), the steering-back maneuverdetermination unit 126 determines in step S302: whether or not themultiplication value Ra1×Ra1′ is negative, that is, whether or not thesigns indicating right and left of the lateral acceleration-based rollangle Ra1 and the lateral acceleration-based roll rate Ra1′ aredifferent from each other; and whether or not the absolute value of thelateral acceleration-based roll rate Ra1′ is equal to or greater thanthe predetermined value Ra1′th (i.e., whether or not an abruptsteering-back maneuver is made). If these conditions in step S302 aresatisfied (S302; Yes), the steering-back maneuver determination unit 126further determines in step S303: whether or not the multiplication valueRa2×Ra2′ is negative, that is, whether or not the signs indicating rightand left of the yaw rate-based roll angle Ra2 and the yaw rate-basedroll rate Ra2′ are different from each other; and whether or not theabsolute value of the yaw rate-based roll rate Ra2′ is equal to orgreater than the predetermined value Ra2′th (i.e., whether or not anabrupt steering-back maneuver is made). If these conditions in step S303are satisfied, the steering-back maneuver determination unit 126determines that an abrupt steering-back maneuver is made. Referring toFIG. 14, the conditions in step S301-S303 are satisfied in the timeperiod from t22 to t24. If these conditions are satisfied, it isdetermined that an abrupt steering-back maneuver is made, which islikely to cause a rollover of the vehicle. For this reason, the secondcounter 128B increases the second count value C2 (S304). Further, if thesecond count value C2 is greater than the upper limit value C2max (S305;Yes), the second counter 128B sets the second count value C2 to theupper limit value C2max (S306). If the second count value C2 is notgreater than the upper limit value C2max (S305; No), the second counter128B sets the increased value as the second count value C2.

On the contrary, if any of the conditions is not satisfied in stepsS301-S303 (S30′-S303; No), that is, when an abrupt steering-backmaneuver is not made, the second counter 128B decreases the second countvalue C2 (S311, see also time period from t24 to t26 of FIG. 14). If thesecond count value C2 is smaller than 0 (S312; Yes), the second countvalue C2 is set to 0 (S313), so that the second count value C2 takes avalue not smaller than 0. If the second count value C2 is not smallerthan 0 (S312; No), the value is set as the second count value C2.

When the second count value C2 is determined by the above steps, thesecond count value C2 is multiplied by a coefficient α2 to obtain thesecond weight coefficient K2 (S307). If the second weight coefficient K2is greater than K2max that is a value smaller than 1 (S308; Yes), thesecond weight coefficient K2 is set to the upper limit value, that isK2max in this embodiment (S309). If the second weight coefficient K2 isnot greater than K2max (S308; No), the value is set as the second weightcoefficient K2.

Returning now to FIG. 7, after the first weight coefficient K1 and thesecond weight coefficient K2 are obtained, the parameter calculationunit 128 calculates the first composition roll angle Ra12 by thefollowing expression (S107), by combining the lateral acceleration-basedroll angle Ra1 and the yaw rate-based roll angle Ra2 using the firstweight coefficient K1.Ra12=K1×Ra2+(1−K1)×Ra1

As seen in FIG. 13, the first composition roll angle Ra12 is based onthe lateral acceleration-based roll angle Ra1; however, if the firstweight coefficient K1 is greater than 0 (e.g., time period from t11 tot15), the first composition roll angle Ra12 reflects the yaw rate-basedroll angle Ra2 which changes with a phase earlier than the lateralacceleration-based roll angle Ra1, and in the time period from t12 tot14, the first composition roll angle Ra12 completely follows the yawrate-based roll angle Ra2.

Thereafter, the parameter calculation unit 128 calculates the secondcomposition roll angle Ra by the following expression (S108), bycombining the first composition roll angle Ra12 and the steeringangle-based roll angle Ra3 using the second weight coefficient K2.Ra=K2×Ra3+(1−K2)×Ra12

As seen in FIG. 14, the second composition roll angle Ra is based on thefirst composition roll angle Ra12; however, if the second weightcoefficient K2 is greater than 0 (e.g., time period from t22 to t26),the second composition roll angle Ra is obtained by combining thesteering angle-based roll angle Ra3 which changes with a phase earlierthan the lateral acceleration-based roll angle Ra1 and the yawrate-based roll angle Ra2 with the first composition roll angle Ra12,and in the time period from t23 to t25, the second composition rollangle Ra approaches the steering angle-based roll angle Ra3.

Returning to FIG. 6, the parameter calculation unit 128 sets the rollangle threshold value Rath in step S400. To be more specific, as seen inFIG. 10, the threshold calculation roll rate calculation unit 128Ecalculates the composition roll rate Ra12′ by the following expression(S401), by combining the lateral acceleration-based roll rate Ra1′ andthe yaw rate-based roll rata Ra2′ using the first weight coefficient K1.Ra12′=K1×Ra2′+(1−K1)×Ra1′

As seen in FIG. 15, the composition roll rate Ra12′ is based on thelateral acceleration-based roll rate Ra1′; however, if the first weightcoefficient K1 is greater than 0 (e.g., time period from t11 to t15),the composition roll rate Ra12′ reflects the yaw rate-based roll rateRa2′ obtained from the yaw rate-based roll angle Ra2 which changes witha phase earlier than the lateral acceleration-based roll angle Ra1, andin the time period from t12 to t14, the composition roll rate Ra12′completely follows the yaw rate-based roll rate Ra2′.

The threshold calculation roll rate calculation unit 128E thencalculates the threshold calculation roll rate (S402) by taking theabsolute value of the composition roll rate Ra12′ as shown in FIG. 16and filtering the obtained value such that the absolute value of thecomposition roll rate Ra12′ decreases less.

Further, the threshold calculation roll rate calculation unit 128Edetermines whether or not the instability level calculated by theinstability level calculation unit 127 is equal to or greater than thepredetermined level Lv. If the instability level is smaller than thepredetermined level Lv (S403; No), the threshold calculation roll rateis set to 0 (S404). If the instability level is not smaller than thepredetermined level Lv (S403; Yes), the threshold calculation roll rateis not changed.

Next, the parameter calculation unit 128 refers to a conversion table ofFIG. 5 for converting between threshold calculation roll rate and rollangle threshold value Rath, and sets the roll angle threshold value Rathfrom the threshold calculation roll rate (S405). Accordingly, as bestseen in FIG. 16, when the threshold calculation roll rate increasesabruptly at the time t31 and in the time period from t32 to t33, theroll angle threshold value Rath decreases abruptly, and when thethreshold calculation roll rate is equal to or greater than γ2 (timeperiod from t33 to t34), the roll angle threshold value is 0.

When the second composition roll angle Ra as the rollover detectionparameter and the roll angle threshold value Rath are obtained asdescribed above, the rollover prevention control unit 120 compares thesecond composition roll angle Ra with the roll angle threshold valuesRath, −Rath, as shown in FIG. 6. If the difference between the secondcomposition roll angle Ra and the positive roll angle threshold valueRath is greater than the predetermined value ΔRath or if the differencebetween the second composition roll angle Ra and the negative roll anglethreshold value −Rath is smaller than the predetermined value −ΔRath(S5; Yes, see also time period from t1 to t7 of FIG. 17), then therollover prevention control flag is turned ON (S6). On the contrary, ifthe difference between the second composition roll angle Ra and thepositive roll angle threshold value Rath is not greater than thepredetermined value ΔRath and if the difference between the secondcomposition roll angle Ra and the negative roll angle threshold value−Rath is not smaller than the predetermined value −ΔRath (S5; No, seealso time period before t1 and time period after t7 of FIG. 17), thenthe rollover prevention control flag is turned OFF. In order not toaffect the setting of the outside wheel target braking force to bedescribed later, the difference ΔRa is set to 0 (S7). The condition fordetermining optimum start timing for the rollover prevention control mayinclude, for example, whether the vehicle speed Vx is equal to orgreater than a predetermined value, and whether the corrected lateralacceleration Ygm is equal to or greater than a predetermined value.

The outside wheel target braking force setting unit 129A sets theoutside wheel target braking force for the rollover prevention control(S500). To be more specific, as seen in FIGS. 11 and 18A, the Pcomponent (proportional component) and I component (integral component)are calculated from the difference ΔRa, and the PI output value isobtained by adding these components (S501). As seen in FIG. 18A, theupper limit value Imax is set for the I component. The outside wheeltarget braking force setting unit 129A then calculates Fout bymultiplying the PI output value by the predetermined correctioncoefficient (S502). For the purpose of better comprehension from FIGS.18A and 18B, the correction coefficient is smaller than 1.

Further, the outside wheel target braking force setting unit 129Acalculates Fout1 as the braking force for the rear wheels; Fout1 islimited (limit controlled) to Foutmax (S503).

The inside wheel target braking force setting unit 129B sets the insidewheel target braking force for performing the rollover preventioncontrol (S600). To be more specific, as seen in FIG. 12, a determinationis made in step S601 as to whether the rollover prevention control hasjust been initiated and the pressure in the wheel cylinder is equal toor lower than a predetermined value (S601). The initiation of therollover prevention control can be determined from the former value andthe present value on the rollover prevention control flag; if the formervalue is OFF and the present value is ON, it can be said that therollover prevention control has just been initiated. The condition as towhether or not the pressure in the wheel cylinder is equal to or lowerthan the predetermined value is optional; however, if the pressure inthe wheel cylinder is not smaller than the predetermined value, it isnot necessary to consider the use time of the braking force applied tothe inside wheel and hence in this embodiment this condition is includedin step S601. If the conditions in step S601 are Satisfied (S601; Yes),a value Tm1 is assigned to a timer Tm to start the timer Tm (S602). Ifthese conditions in step S601 are not satisfied (S601; No), adetermination is made as to whether or not the rollover preventioncontrol has been finished or the turning direction of the vehicle CR hasbeen changed to the opposite direction (S603). If one of the conditionsin step S603 is satisfied (S603; Yes), it can be said that the rolloverprevention control has been finished or the inside wheels and theoutside wheels have been changed to each other after the driver'ssteering-back maneuver, and hence a value 0 is assigned to the timer Tmto reset the timer Tm (S604). If one of the conditions in step S603 isnot satisfied (S603; No), the timer Tm starts counting down (S605). Inorder to avoid the timer Tm having a negative value after the stepsS602, S604, and S605, the values Tm and 0, whichever is greater, isassigned to the timer Tm (S606).

Next, the inside wheel target braking force setting unit 129B sets theinside wheel target braking force in accordance with the value of thetimer Tm (see also FIG. 19 for setting the inside wheel target brakingforce).

First, a determination is made as to whether or not the rolloverprevention control flag is ON (S607). If the rollover prevention controlflag is not ON (S607; No), then the inside wheel target braking forcesfor the front wheels (Fin) and for the rear wheels (Fin1) are set to 0(S608). If the rollover prevention control flag is ON (S607; Yes), adetermination is made as to whether or not the value of the timer Tm isgreater than 0 (S609). If the value of the timer Tm is greater than 0(S609; Yes), the inside wheel target braking forces for the front wheels(Fin) and the rear wheels (Fin1) are set to the first predeterminedvalue B1 (S610) because the rollover prevention control is at an initialstage. On the contrary, if the value of the timer Tm is not greater than0 (S609; No), the inside wheel target braking forces for the frontwheels (Fin) and the rear wheels (Fin1) are set to the secondpredetermined value B2 (S611) because the rollover prevention control isnot at the initial stage.

Further, the inside wheel target braking force setting unit 129Bcompares a multiplication value obtained by multiplying the outsidewheel target braking force Fout for the front wheels by the coefficientβ with the previously obtained inside wheel target braking force Fin forthe front wheels, and the smaller one is newly set as the inside wheeltarget braking force Fin (S612). Therefore, if the inside wheel targetbraking force Fin (the first predetermined value B1 or the secondpredetermined value B2) is smaller than the multiplication valueobtained by multiplying the outside wheel target braking force by thecoefficient β, the first predetermined value B1 or the secondpredetermined value B2 are set as the inside wheel target braking forceFin. If the inside wheel target braking force Fin (the firstpredetermined value B1 or the second predetermined value B2) is notsmaller than the multiplication value, the value obtained by multiplyingthe outside wheel target braking force by the coefficient β is set asthe inside wheel target braking force Fin.

Similarly, a multiplication value obtained by multiplying the outsidewheel target braking force Fout1 for the rear wheels by the coefficientβ is compared with the inside wheel target braking force Fin1 for therear wheels, and the smaller one is newly set as the inside wheel targetbraking force Fin1 (S612).

As described above, the target braking force Fout for the front-sideoutside wheel, the target braking force Fout1 for the rear-side outsidewheel, the target braking force Fin for the front-side inside wheel, andthe target braking force Full for the rear-side inside wheel areobtained. Thereafter, according to the steps S14-S17 in FIG. 6, thetarget braking forces for the respective wheels are set based on whetherthe vehicle CR hum right or left.

To be more specific, the ROM control braking force setting unit 129determines whether or not the second composition roll angle Ra isgreater than 0 (S14). If the second composition roll angle Ra is greaterthan 0, it is determined that the vehicle CR is turning left (S14; Yes),and the ROM control braking force setting unit 129 sets the targetbraking force F_(FR) for the front-side right wheel (the outside wheel)to Fout, the target braking force F_(RR) for the rear-side right wheelto Fout1, the target braking force F_(FL) for the front-side left wheel(the inside wheel) to Fin, and the target braking force F_(RL) for therear-side left wheel to Fin1 (S15). On the contrary, if the secondcomposition roll angle Ra is not greater than 0, it is determined thatthe vehicle CR is turning right (S14; No), and the ROM control brakingforce setting unit 129 sets the target braking force F_(FR) for thefront-side right wheel (the inside wheel) to Fin, the target brakingforce F_(RR) for the rear-side right wheel to Fin1, the target brakingforce F_(FL) for the front-side left wheel (the outside wheel) to Fout,and the target braking force F_(RL) for the rear-side left wheel toFout1 (S16).

The target braking forces set for the wheels as described above and usedfor the rollover prevention control are shown in FIGS. 20A-20D.

When the target braking forces for the wheels are set by the abovesteps, the rollover prevention control unit 120 outputs the targetbraking forces for the respective wheels to the target braking forcesetting unit 130. The target braking force setting unit 130 thencompares the target braking forces output from the behavior control unit110 and the target braking forces output from the rollover preventioncontrol unit 120, and sets the greater ones as the target brakingforces. The target braking force setting unit 130 outputs instructionsto the valve drive unit 140 and the motor drive unit 150 such that thethus obtained target braking forces are applied to the respectivewheels. Accordingly, braking forces are applied at the same time to allthe four wheels (S17).

As described above, according to the brake fluid pressure controlapparatus A for a vehicle in this embodiment, if a rollover tendency ofthe vehicle is detected, braking forces are applied to the four wheelsrespectively at the same timing with the brake applications beinginitiated at the same starting point. Further, irrespective of whetheror not the driver is likely to make a steering-back maneuver, brakingforces are always applied to the four wheels at the same timing. Namely,since braking force applications for the rollover prevention control areinitiated at the same timing for the right and left wheels arranged onthe same axle, the vehicle speed can be effectively reduced by makinguse of the braking forces applied to the inside wheels. This can alsocontribute to suppress the rollover tendency of the vehicle. If thedriver makes a steering-back maneuver, since preparatory brake isapplied from the beginning of the rollover prevention control for thewheels which are on the inside before the steering-back maneuver,sufficiently large braking forces can be applied promptly to the wheelswhich are on the outside after the steering-back maneuver (i.e., wheelswhich are on the inside before the steering-back maneuver). Therefore,if the driver makes a steering-back maneuver, the brake fluid pressuresin the outside wheels are increased promptly to effectively prevent arollover of the vehicle.

According to the brake fluid pressure control apparatus A, since theoutside wheel target braking force is set based on the difference ΔRabetween the roll angle (the second composition roll angle Ra) indicatingthe rollover tendency and the roll angle threshold value Rath, it ispossible to perform the control in accordance with the magnitude of therollover tendency.

Since braking forces are applied to both front and rear wheels W, thevehicle speed of the vehicle CR can be effectively reduced to suppressthe rollover tendency. Further, since the outside wheel target brakingforce for the rear wheels is limited to the predetermined value Foutmax,it is possible to suppress the unstable driving condition of the vehicleCR due to slippage of the rear wheels.

According to the brake fluid pressure control apparatus A, since theinside wheel target braking force is set to a greater value at aninitial stage of the rollover prevention control to promptly reduce thevehicle speed, the vehicle CR can be stabilized.

Further, according to the brake fluid pressure control apparatus A inthis embodiment, if the driver makes an abrupt steering maneuver, thesecond composition roll angle Ra (the first composition roll angle Ra12)is calculated using the yaw rate-based roll angle Ra2 which changes witha phase earlier than the lateral acceleration-based roll angle Ra1equivalent to the actual roll angle, and the obtained second compositionroll angle Ra is set as the rollover detection parameter. Therefore, ifthere is a high possibility of rollover of the vehicle, such a highpossibility can be detected earlier and the rollover prevention controlcan be initiated at an earlier timing. If the driver makes an abruptsteering-back maneuver, the second composition roll angle Ra iscalculated using the steering angle-based roll angle Ra3 which changeswith a phase earlier than the lateral acceleration-based roll angle Ra1and the yaw rate-based roll angle Ra2, and the obtained secondcomposition roll angle Ra is set as the rollover detection parameter.Therefore, if there is a higher possibility of rollover of the vehicle,such a higher possibility can be detected earlier and the rolloverprevention control can be initiated promptly to improve the rolloverprevention effect.

The above advantageous effects will be described with reference to FIG.21. As seen in the figure, when the first weight coefficient K1 and thesecond weight coefficient K2 are not 0 (i.e., time period from t41 tot44, particularly at time period from t42 to t43), the secondcomposition roll angle changes with a phase earlier than the lateralacceleration-based roll angle and the first composition roll angle asseen in the graph showing the rollover detection parameter. Accordingly,the rollover prevention control can be executed promptly and the caliperpressure in the wheel brake FL increases quickly (in the graph, thesolid line indicates the embodiment according to the present invention,and dashed line indicates a comparative embodiment). Therefore, as seenin the graph showing wheel lifting amount, the rollover preventioncontrol according to the embodiment can reduce the wheel lifting amountof the vehicle as compared with the control made by the comparativeembodiment.

Further, according to the brake fluid pressure control apparatus A inthis embodiment, the parameter calculation unit 128 sets the roll anglethreshold value Rath to a smaller value with an increase in thethreshold calculation roll rate. Therefore, in the case where thethreshold calculation roll rate is greater, the roll angle thresholdvalue Rath is smaller accordingly, so that the lateralacceleration-based roll angle Ra1 as the rollover detection parameter isapt to be greater than the roll angle threshold value Rath. As a result,the rollover prevention control can be initiated promptly under such acondition that the vehicle is apt to roll over due to rapidly increasedroll angle. This can improve the stability while driving the vehicle

According to the brake fluid pressure control apparatus A in thisembodiment, the threshold calculation roll rate is set to zero if theinstability level is smaller than the predetermined value Lv, with theresult that the roll angle threshold value Rath increases to preventunnecessary rollover prevention control.

Further, according to the brake fluid pressure control apparatus A inthis embodiment, if the driver makes an abrupt steering maneuver, thecomposition roll rate Ra12′ is calculated using the yaw rate-based rollrate Ra2′ which is a rate of change of the yaw rate-based roll angleRa2; the yaw rate-based roll angle Ra2 changes with a phase earlier thanthe lateral acceleration-based roll angle Ra1 equivalent to the actualroll angle. And based on the thus obtained composition roll rate Ra12′,the threshold calculation roll rate and the roll angle threshold valueRath are calculated. Therefore, if there is a high possibility ofrollover of the vehicle, such a high possibility can be detected earlierand the rollover prevention control can be initiated at an earliertiming.

Although the present invention has been described in detail withreference to an exemplary embodiment, the present invention is notlimited to this specific embodiment. It is to be understood thatmodifications and changes may be made to any of the specificconfigurations without departing from the scope of the appended claims.

For example, in the above embodiment, the lateral acceleration-basedroll angle Ra1 is exemplified as the first roll angle and the yawrate-based roll angle roll angle Ra2 is exemplified as the second rollangle. However, the lateral acceleration-based roll angle Ra1 as thefirst roll angle and the steeling angle-based roll angle Ra1 as thesecond roll angle may be combined using the first weight coefficient K1to obtain the rollover detection parameter.

In the above embodiment, for the purpose of accurate determination ofthe steering-back maneuver, the steering-back determination unit 126determines for both of the lateral acceleration-based roll angle Ra1 andthe yaw rate-based roll angle Ra2 whether or not the signs indicatingright and left of the roll angle and the roll rate are different fromeach other, and whether or not the absolute value of the roll rate isequal to or greater than the predetermined value. However, thesteering-back determination unit 126 may determine the abruptsteering-back maneuver based on either one of the first roll angle andthe second roll angle. To be more specific, the steering-backdetermination unit 126 may determine that the driver makes an abruptsteering-back maneuver: (a) if at least one of the first roll angle andthe second roll angle has a different sign with respect to the signsindicating right and left of the roll angle and the roll rate; and (b)if the absolute value of the roll rate that satisfies the abovecondition (a) is equal to or greater than a predetermined value.

In the above embodiment, for the purpose of executing the rolloverprevention control at an earlier timing, the second composition rollangle Ra is used as the rollover detection parameter. However, thepresent invention is not limited to this specific embodiment, and thefirst composition roll angle Ra12 as described above, the roll angleobtained from the lateral acceleration, or the roll angle detected fromthe roll angle sensor may be used as the rollover detection parameter.

In the above embodiment, when an abrupt steering maneuver is not made,the first weight coefficient K1 is set to 0; however, the first weightcoefficient K1 may be set to a value greater than 0. Further, the upperlimit value of the first weight coefficient K1 is 1 in the aboveembodiment. However, the upper limit value of the first weightcoefficient K1 may be a value smaller than 1. In the above embodiment,when an abrupt steering-back maneuver is not made, the first weightcoefficient K1 is set to 0; however, the first weight coefficient K1 maybe set to a value greater than 0.

In the above embodiment, the first weight coefficient setting unit 128Cand the second weight coefficient setting unit 128D determine the weightcoefficients, respectively, by multiplying each of the count values bythe corresponding coefficient. However, the relationship between thecount values and the weight coefficients may be stored in advance in atable, and the weight coefficients may be determined from the countvalues based on the table.

In the above embodiment, the steering maneuver determination unit 125determines that the driver makes an abrupt steering maneuver if theabsolute value of the steering wheel turning speed is equal to orgreater than the predetermined value, and if the filtered absolute valueof the lateral acceleration resulting from the filtering process bywhich a decrease of the absolute value of the lateral acceleration isretarded is equal to or greater than the predetermined value. However,the steering maneuver determination unit 125 may determine that thedriver makes an abrupt steering maneuver if the absolute value of thesteering wheel turning speed is equal to or greater than thepredetermined value and the absolute value of the lateral accelerationis equal to or greater than the predetermined value.

In the above embodiment, the parameter calculation unit 128 calculatesthe threshold calculation roll rate which is a rate of change of theroll angle of the vehicle CR, and sets the roll angle threshold value(the parameter threshold value) to a smaller value with an increase inthe filtered absolute value of the threshold calculation roll rateresulting from the filtering process by which a decrease of the absolutevalue of the threshold calculation roll rate is retarded. However, theparameter threshold value may be obtained and set using an unfilteredthreshold calculation roll rate.

In the above embodiment, the lateral acceleration-based roll rate Ra1′as the first roll rate and the yaw rate-based roll rate Ra2′ as thesecond roll rate are exemplified. However, the lateralacceleration-based roll rate Ra1 as the first roll rate, and thesteering angle-based roll rate as the second roll rate, which iscalculated from the steering angle, may be combined using the firstweight coefficient K1 to obtain the rollover detection parameter.

In the above embodiment, the roll angle threshold value is set to agreater value so that the rollover prevention control is not promptlyinitiated. This is because if the value of the roll rate is smaller, thevehicle is less likely to roll over. However, according to the presentinvention, the roll angle threshold value may be a fixed value.

In the above embodiment, the outside wheel target braking force is setby means of the PI control. However, the detailed setting method for theoutside wheel target braking force is not limited to this specificmethod, and other methods may be adopted.

In the above embodiment, braking forces are applied to both front wheelsand rear wheels during the rollover prevention control. However, brakingforces may be applied to either one of the front wheels and the rearwheels.

What is claimed is:
 1. A brake fluid pressure control apparatus forexecuting rollover prevention control, in which a brake is applied to atleast one wheel of a vehicle at a timing when a rollover tendency of thevehicle is detected through a rollover detection parameter while thevehicle is turning, the brake fluid pressure control apparatuscomprising: a parameter calculation unit that calculates the rolloverdetection parameter; and a steering maneuver determination unit thatdetermines whether or not an abrupt steering maneuver is made, whereinthe parameter calculation unit calculates a first composition roll angleas the rollover detection parameter by combining, at a predeterminedfirst composition roll angle weight assignment ratio, a first roll anglewith a second roll angle, the first roll angle being equivalent to anactual roll angle and the second roll angle being obtained using aparameter which changes with a phase earlier than the first roll angle,and wherein the parameter calculation unit calculates the firstcomposition roll angle by changing the first composition roll angleweight assignment ratio such that a weight of the second roll angle ishigher when the steering maneuver determination unit determines that theabrupt steering maneuver is made than when the steering maneuverdetermination unit determines that the abrupt steering maneuver is notmade.
 2. The brake fluid pressure control apparatus according to claim1, further comprising a steering-back maneuver determination unit thatdetermines whether or not an abrupt steering-back maneuver is made, andwherein the parameter calculation unit calculates a second compositionroll angle as the rollover detection parameter by combining, at apredetermined second composition roll angle weight assignment ratio, athird roll angle with the first composition roll angle, the third rollangle being obtained from a parameter which changes with a phase earlierthan the first roll angle and the second roll angle, and wherein theparameter calculation unit calculates the second composition roll angleby changing the second composition roll angle weight assignment ratiosuch that a weight of the third roll angle is higher when thesteering-back maneuver determination unit determines that the abruptsteering-back maneuver is made than when the steering-back maneuverdetermination unit determines that the abrupt steering-back maneuver isnot made.
 3. The brake fluid pressure control apparatus according toclaim 1, wherein the steering maneuver determination unit determinesthat the abrupt steering maneuver is made, if at least one of thefollowing conditions is satisfied: (a) an absolute value of a steeringwheel turning speed is equal to or greater than a predetermined steeringwheel turning speed value, and an absolute value of a lateralacceleration is equal to or greater than a predetermined lateralacceleration value; and (b) the absolute value of the steering wheelturning speed is equal to or greater than the predetermined steeringwheel turning speed value, and a filtered absolute value of the lateralacceleration resulting from a filtering process by which a decrease ofthe absolute value of the lateral acceleration is retarded is equal toor greater than the predetermined lateral acceleration value.
 4. Thebrake fluid pressure control apparatus according to claim 1, wherein theparameter calculation unit comprises a first counter that increases afirst count value if the steering maneuver determination unit determinesthat the abrupt steering maneuver is made, and to decrease the firstcount value if the steering maneuver determination unit determines thatthe abrupt steering maneuver is not made, and a first weight coefficientsetting unit configured to set a first weight coefficient, which isequivalent to the weight of the second roll angle, in accordance withthe first count value and in a range equal to or smaller than apredetermined upper limit value, and wherein the first counter increasesthe first count value even after the first weight coefficient reachesthe predetermined upper limit value.
 5. The brake fluid pressure controlapparatus according to claim 2, wherein signs of values are defined byassigning first signs respectively to a value of a steering angle whenthe steering wheel is turned left, values of a lateral accelerationacting on the vehicle and a roll angle exhibited while the vehicle isstably turning left, and a value of a roll rate exhibited when the rollangle takes a greater value due to a left turn of the vehicle, and byassigning second signs respectively to a value of a steering angle whenthe steering wheel is turned right, values of the lateral accelerationacting on the vehicle and the roll angle exhibited while the vehicle isstably turning right, and the value of the roll rate exhibited when theroll angle takes a greater value due to a right turn of the vehicle, andthe steering-back maneuver determination unit determines that the abruptsteering-back maneuver is made if all of the following conditions aresatisfied: (1) one of the values of the steering angle and the lateralacceleration has the first sign, while the other one of the values hasthe second sign; (2) one of the values of the first roll angle and afirst roll rate calculated from the first roll angle has the first sign,while the other one of the values has the second sign, or/and one ofvalues of the second roll angle and a second roll rate calculated fromthe second roll angle has the first sign, while the other one of thevalues has the second sign; and (3) an absolute value of the first rollrate that satisfies the above condition (2) is equal to or greater thana predetermined first roll rate value or/and an absolute value of thesecond roll rate that satisfies the above condition (2) is equal to orgreater than a predetermined second roll rate value.
 6. The brake fluidpressure control apparatus according to claim 5, wherein the parametercalculation unit comprises a second counter that increases a secondcount value if the steering-back maneuver determination unit determinesthat the abrupt steering-back maneuver is made, and to decrease thesecond count value if the steering-back maneuver determination unitdetermines that the abrupt steering-back maneuver is not made, and asecond weight coefficient setting unit configured to set a second weightcoefficient, which is equivalent to the weight of the third roll angle,in accordance with the second count value and in a range equal to orsmaller than a predetermined upper limit value, and wherein the secondcounter increases the second count value even after the second weightcoefficient reaches the predetermined upper limit value.
 7. The brakefluid pressure control apparatus according to claim 1, wherein the firstroll angle is calculated from a lateral acceleration, and the secondroll angle is calculated from a yaw rate.
 8. The brake fluid pressurecontrol apparatus according to claim 2, wherein the third roll angle iscalculated from a steering angle.